The short answer for a very small, shy fish
A celestial pearl danio travels best in a thick fish bag holding about one-third of its home tank water and two-thirds air or oxygen, sealed, doubled, and set upright in a dark insulated box. This species is tiny, timid, and used to cool, planted water, so the whole plan for a long journey is built around three things: stable cool temperature, darkness that lets the fish rest instead of darting, and enough oxygen above a small volume of clean water. A hard outer box also protects the bag from being squeezed by luggage shifting around it.
Preparing the danios the day before
Fasting is the preparation that matters most for celestial pearl danios on a long journey. Stop feeding a full day before departure so the fish produce very little waste inside a bag where nothing filters it out. Use that day to confirm the group looks settled, with males showing their spotted flanks and females swimming calmly rather than hiding constantly. Catch them gently with a soft fine net or a cup, because chasing these thumbnail-sized fish around plants exhausts them before the trip even starts. Bag schoolmates together, since a lone danio separated from its group stresses far more than one travelling with familiar companions.
Packing the bag and box for danios
Water taken straight from the danios' own tank is the right water for the bag, because a sudden chemistry change adds strain a long journey does not need. Fill the inner bag about one-third full, tie or band it with the largest air pocket you can manage, then slide it into a second bag tied the opposite way round so a leaking corner cannot drain the first. Stand the bags upright in the insulated box, packed snugly with soft material so they cannot topple and roll. Keep the box somewhere cool and shaded in the vehicle, never against a heater vent and never in direct sun, because this cool-water species suffers quickly when a small bag warms up.
During the journey, leave the box closed
Opening the box to check on celestial pearl danios during a long journey does more harm than good. Every opening floods shy fish with light, drops the temperature cushion around the bags, and tempts you to feed them, which only fouls the water faster. Instead, plan stops around the box rather than the fish: keep it level, keep it out of a hot parked car, and carry it by hand over rough ground instead of leaving it to bounce in a trunk. If the trip runs far longer than expected, darkness and cool stability still protect the fish better than a hurried water change in unfamiliar water at a rest stop.
Settling danios at the destination
Floating the sealed bag in the destination tank is the first settling step for celestial pearl danios, letting bag and tank temperatures meet slowly. Once they match, open the bag and add small amounts of tank water over time before releasing the fish, rather than tipping them straight in. Keep lights low for the first hours so the group can find cover among plants, and hold off feeding until they are exploring instead of huddling. Watch for fish that stay clamped and motionless or gasp at the surface, and seek advice from an aquatic veterinarian or experienced fish shop if that behaviour does not ease once conditions are stable. Keeping the danios' temperature, feeding, and water notes together in a KinsLe Pet Passport means the person receiving them works from the same details you packed them with.